Benzo[b]fluorenes Formed in the Thermal Cyclization of 3-Ene-1,6-diynes

Abstract

A three-step preparation of the benzofluorene core is presented. The last step involves thermal cyclization of 3 -ene-1,6 -diyne (7) leading to the formation of four benzofluorene derivatives, one of which has been investigated by X-ray analysis. The harsh thermal conditions indicate that the cyclization of 7 might not proceed via a biradical intermediate as would be anticipated by a mechanistic proposal from Ueda.

Dynamic effects in the didehydro‐Diels‐Alder (DDDA) reaction of enyne‐ketoenes: 50% stepwise bond formation in spite of concerted transition state

“…We synthesized the enyne‐ketoene 1 according to the reported procedure and investigated its thermal behavior by heating it at 160°C in toluene for 1 hour. While the reaction mixture was found to be more complicated than anticipated, the expected product 2 was readily identified by a sharp singlet at 8.25 ppm (in CDCl 3 ) matching the position in the literature reported 1 H NMR of 2 . Strikingly, at 100°C (for 7 days) the thermal reaction of 1 in PhMe‐ d 8 proceeded in a much cleaner way affording 2 in 48% yield.…”

Kinetic Isotope Effects in the Thermal C2−C6 Cyclization of Enyne-allenes: Experimental Evidence Supports a Stepwise Mechanism

“…After the mixture had been stirred for 4 h at room temperature, a solution of 4 (1.00 g, 3.48 mmol) in dry THF (10 mL) was added dropwise. 5 After stirring for 14 h at room temperature, the reaction mixture was quenched with saturated ammonium chloride solution (25 mL). The aqueous layer was extracted with diethyl ether (2 x 25 mL).…”

Fluoro‐Ene Reaction Versus [2+2] Cycloaddition in the Thermal C2–C6 Cyclization of Enyne‐Allenes: An Experimental and Theoretical Investigation

“…Accordingly, Sonogashira cross‐coupling of o ‐iodobenzaldehyde with TIPS‐C≡CH afforded aldehyde 2 23. The propargyl alcohol 3 was obtained in moderate yield by nucleophilic addition of 1‐perfluorohexyne to aldehyde 2 by using the Grignard methodology 24. The reaction of 3 with acetic anhydride at room temperature in the presence of 4‐( N , N ‐dimethylamino)pyridine (DMAP) provided propargyl acetate 4 25.…”

“…All solvents were distilled prior to use for column chromatography. 3,3,4,4,5,5,6,6,6‐Nonafluorohex‐1‐yne,34 2‐(2‐phenylethynyl)benzaldehyde,24 2‐[2‐( p ‐tolyl)ethynyl]benzaldehyde,1b 2‐(3,3‐dimethylbut‐1‐ynyl)benzaldehyde,35 and 1‐[2‐(2‐phenylethynyl)phenyl]pentan‐1‐ol8b were synthesized according to literature procedures.…”